NASA's Magellan spacecraft will dip into the atmosphere of Venus beginning May 25 in a first-of-its-kind maneuver lowering the spacecraft's orbit to start a new experiment.

The technique, called "aerobraking," will use atmospheric drag to slow the spacecraft and circularize its orbit. Currently Magellan is looping around Venus in a highly elliptical orbit.

Before the experiment starts, Magellan will have completed all of its primary mission goals to map Venus with radar and collect gravity data.

"This aerobraking technique has never been used before on a NASA planetary mission," said Douglas Griffith, Magellan project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

"Magellan has been highly successful in completing all of its primary mission goals," said Alphonso V. Diaz, deputy associate administrator for NASA's Office of Space Science. "The new orbit will enhance the scientific return from what is already one of NASA's most productive space science missions."

According to Griffith, aerobraking is the only way to make such a large change in Magellan's orbit because the spacecraft does not have enough thruster fuel onboard for the change. "Although aerobraking creates some risk of losing the spacecraft, the scientific benefits make the risk worthwhile."

The main benefit of changing the orbit is to make better measurements of Venus's gravity field, particularly at latitudes near the planet's poles, said Magellan Project Scientist Dr. R. Stephen Saunders of JPL.

For the past eight months, Magellan has been collecting data on Venus's gravity. However, measurements from the current elliptical orbit are blurred at high latitude by the height of the spacecraft above the surface -- about 2,100 kilometers (1,300 miles) near the north pole, and 2,800 kilometers (1,700 miles) near the south pole.

Scientists also hope to study Venus's atmosphere using data collected during the aerobraking experiment itself. "And another objective is to gain the engineering experience that may allow future missions to use aerobraking to enter planetary orbit or to change orbit without using large thrusters," said Griffith.

Launched in May 1989, Magellan will complete its fourth 243day orbital cycle at Venus on May 25. During each of the eight month cycles, Magellan orbits from north to south while the planet turns slowly once underneath the spacecraft.

During earlier cycles, Magellan used its radar to map Venus's surface with resolution as fine as 75 meters (250 feet), and the elevation, slope, radar reflectivity and radar emissivity over more than 98 percent of the planet.

In the upcoming maneuver, flight controllers hope to lower the spacecraft from its current orbit with a low point near 170 kilometers (100 miles) and high point of 8,500 kilometers (5,300 miles). The target orbit is about 200 by 600 kilometers (125 by 375 miles). This change would alter the time taken by each orbit from 3-1/4 hours to about 90 minutes.

The aerobraking experiment will start May 25 at 17:30 UTC (10:30 a.m. Pacific time or 1:30 p.m. Eastern) when the spacecraft makes the first maneuver to lower the low point of its orbit down into Venus's atmosphere. By carefully controlling the orbit altitude, the drag and heat generated on the spacecraft will be kept within tolerable limits.

Completing the change in orbit down to the final target will take about 80 days. The short period of drag on each orbit, a few minutes at the start to about 20 minutes near the end, will lower the high point by about 10 kilometers (6 miles) on every orbit.

According to Saunders, measuring Venus's gravity field permits scientists to measure the pattern of heavier and lighter regions under the planet's surface. It is the only technique currently possible to look inside Venus, and provides information like that gained using seismometers to probe inside a planet. Similar measurements on Earth helped reveal plate tectonics, Earth's fundamental geologic process.

"Without better measurements from a lower orbit, it would remain very hard to understand Venus's internal geology, and why it is so different from Earth," said Saunders.